A known image forming apparatus includes a supporting portion configured to support a sheet inserted from an opening formed in a casing, and a cover configured to open and close the opening, and to support, with the opening open, a portion of the sheet on the supporting portion when the portion is disposed outside the casing. The cover includes a regulation guide configured to regulate a position of an upstream end of a sheet in a sheet conveying direction. The regulation guide is configured to slidably move in the sheet conveying direction relative to the cover, and to be removably attached to the cover. Accordingly, the regulation guide may be removed from the cover and reversed to regulate positions of upstream ends of sheets of different sizes.